fix the "missing scrollbars" problem

src/widgets/scrollarea.cpp

@@ -33,6 +33,13 @@ bool scrollarea::has_scrollbar() const
 	return shown_size_ < full_size_ && scrollbar_.is_valid_height(location().h);
 }
 
+handler_vector scrollarea::handler_members()
+{
+	handler_vector h;
+	h.push_back(&scrollbar_);
+	return h;
+}
+
 void scrollarea::update_location(SDL_Rect const &rect)
 {
 	SDL_Rect r = rect;

src/widgets/scrollarea.hpp

@@ -32,6 +32,7 @@ public:
 	virtual void hide(bool value = true);
 
 protected:
+	virtual handler_vector handler_members();
 	virtual void update_location(SDL_Rect const &rect);
 	virtual void handle_event(const SDL_Event& event);
 	virtual void process_event();

src/widgets/scrollbar.cpp

@@ -54,6 +54,14 @@ scrollbar::scrollbar(CVideo &video)
 	}
 }
 
+handler_vector scrollbar::handler_members()
+{
+	handler_vector h;
+	h.push_back(&uparrow_);
+	h.push_back(&downarrow_);
+	return h;
+}
+
 void scrollbar::update_location(SDL_Rect const &rect)
 {
 	int uh = uparrow_.height(), dh = downarrow_.height();

src/widgets/scrollbar.hpp

@@ -70,6 +70,7 @@ public:
 	void scroll_up();
 
 protected:
+	virtual handler_vector handler_members();
 	virtual void update_location(SDL_Rect const &rect);
 	virtual void handle_event(const SDL_Event& event);
 	virtual void process_event();